1. Field of the Invention
This invention relates to a method and a device for electromagnetic stirring in a continuous steel casting process arranged to withdraw a plurality of cast pieces of steel simultaneously in parallel through pairs of opposing pinch rolls (hereinafter will be called a double casting type continuous casting process).
2. Description of the Prior Art
Cast pieces of steel obtained through a continuous casting process are in most cases supplied to subsequent rolling processes as rolling materials. Generally, in integrated steel making plants or electric furnace steel making works of a large scale, where comprehensive manufacture of steel materials is performed, a plurality of rolling mills are usually equipped. The rolling materials to be processed at such plants or works include various kinds of materials such as slabs, blooms, billets, etc. Further, the progresses made in the continuous casting technology has resulted in an increased productivity of continuous casting machines. To utilize the increased productivity to a full extent, it is desirable to have various kinds of rolling materials such as slabs, blooms and billets supplied from a single continuous casting machine. In view of this, a double casting type continuous casting process has been put in practice to withdraw a plurality of cast pieces simultaneously in parallel through pairs of opposing pinch rolls.
Meanwhile, in the continuous casting process, it has long been known to carry out electromagnetic stirring to prevent the occurrence of dendrite and the segregation of impurities within the cast steel pieces in a liquidus-solidus state such as slabs or blooms by controlling solidification of the slabs or blooms with the stirring operation as disclosed, for example, in U.S. Pat. No. 3,656,537 and in Japanese Utility Model Publication No. Sho 52-52893.
In carrying out the electromagnetic stirring in the double casting type continuous casting process, installation of an electromagnetic stirring device for every cast piece presents a problem, because there are a plurality of cast pieces drawn through the pairs of opposing pinch rolls. More specifically, widening the space distance between one cast piece and another for installing one unit of the electromagnetic stirring device for each of them necessitates use of longer rolls for supporting cast pieces. This is not desirable from the aspect of economy. Besides, in the case of the double casting type continuous casting process, cast pieces of different shapes such as slabs and blooms are cast in different numbers according to their shapes, for example, in such a way as to simultaneously cast one slab and two blooms all together. Therefore, arrangements to provide one unit of the electromagnetic stirring device for every cast piece necessitate alteration of these units of the electromagnetic stirring device every time the shape and the number of the cast pieces to be passed through the same pinch rolls are changed. Such arrangements, therefore, require a long period of time for mounting and dismounting the electromagnetic stirring devices. The length of the obstraction time incidental to alteration of the shape of the cast pieces lowers the rate of operation of the double casting type continuous casting apparatus. Such arrangements are therefore uneconomical also in that respect.